Unit Discovery

The Wolfram Language's units system includes thousands of different quantities, including units, physical constants, and (RefLink[IndependentUnit, paclet:ref/IndependentUnit]) expressions. Supported units include all those specified by (WebLink[NIST Special Publication 811, http://www.nist.gov/pml/pubs/sp811/index.cfm]).

Many quantities have multiple common names, which can make them difficult to find in a long list of possible units. The Wolfram Language's built-in unit interpretation system allows you to specify units and physical constants using natural language input.

Use free-form linguistic inline input to make a (RefLink[Quantity, paclet:ref/Quantity]) expression:

LinguisticAssistant//InputForm

The electric constant is also commonly referred to as "vacuum permittivity" or "permittivity of free space":

QuantityUnit[LinguisticAssistant]

You can use both (RefLink[WolframAlpha, paclet:ref/WolframAlpha]) and (RefLink[Quantity, paclet:ref/Quantity]) to discover different units and physical constants using their various common names and abbreviations.

Wolfram|Alpha Integration

One method of unit discovery in the Wolfram Language is the (RefLink[WolframAlpha, paclet:ref/WolframAlpha]) function and the related free-form input methods it provides.

Use the (RefLink[WolframAlpha, paclet:ref/WolframAlpha]) function with "MathematicaResult" format to get the expression as output:

WolframAlpha["planck's constant","MathematicaResult"]//InputForm

The >> assumptions button allows you to disambiguate inputs that could have multiple interpretations:

LinguisticAssistant

Use free-form linguistics inline to convert 0.9 times the speed of light into meters per second:

UnitConvert[LinguisticAssistant,"Meters"/"Seconds"]

Get metabolic information about snorkeling as a (RefLink[Quantity, paclet:ref/Quantity]) expression from Wolfram|Alpha:

data=WolframAlpha["snorkeling",{{"Properties:PhysicalActivity",1},"ComputableData"}];

TableForm[data]

InputForm/@data[[All,2]]

Quantity

(RefLink[Quantity, paclet:ref/Quantity]) will also automatically try and interpret unrecognized units. Once a unit has been interpreted, it will be cached so that subsequent evaluations will use the same result.

Interpret the unit and magnitude associated with Earth's gravity:

Quantity["earth's gravity"]//AbsoluteTiming

Use the cached result to convert Earth's gravity to its SI base units:

UnitConvert[Quantity["earth's gravity"]]//AbsoluteTiming

Unit interpretation requires internet connectivity, and can entail additional evaluation time. If speed is a concern, it is advisable to use the canonical unit specification, which can be found using (RefLink[InputForm, paclet:ref/InputForm]).

AbsoluteTiming[UnitConvert[Quantity[3,"meters"],"Imperial"]]

InputForm[Quantity["3 m"]]

(RefLink[ClearSystemCache, paclet:ref/ClearSystemCache]) can be used to purge any cached (RefLink[Quantity, paclet:ref/Quantity]) expressions:

ClearSystemCache[];
AbsoluteTiming[UnitConvert[Quantity[3,"Meters"],"Imperial"]]